Many of the quiet and power-efficient PCs are very small, but I don't really care what the size is, so I don't want to pay extra or sacrifice performance to get something far smaller than I need.

I want a PC which I can leave on 24/7 without worrying about running costs, but which can play back audio bit-perfectly (external DAC is fine, but decent PCI sound card would be better), and play full HD content (e.g. BBC HD, which is H.264 1080i50 with MBAFF). Needs IEE1394, a way of getting DVB-S2 into it (implies another PCI slot!), and some kind of PC monitor and TV out.

Ideally, it would be able to handle video editing and complex AVIsynth scripts, but wouldn't use many Watts of power when sitting idle or doing less intensive tasks.

Are you looking for pre-assembled systems? Otherwise you can build a very silent system with standard components. Add a decent soundcard/dvb-card and your set. The most important part would be a giant cpu cooler and a quiet power supply. Depending on your needs you add a SSD or some 5600rpm drive in a decoupled QuietDrive.

Just a word of caution on 24/7. I have a quiet (not silent - SSDs are very expensive) computer, which I intended to keep on all the time for general use and music playback through an external soundcard/DAC (no video). It is quiet and works pretty well, except the always on thing. At least in my combination of hardware components, drivers and system (Vista) after roughly 3-4 days of uptime certain operations (mostly display) become sluggish leading to droputs in audio stream, and the only thing that helps is a restart.

Oh, one more thing. If you want quite, you may want to pay special attention to what keyboard you get. Most of them are very noisy, it took me some research time to find one that is resonably quiet still being comfortable, and reasonably priced.

This was purchased back in February, so better options are probably available, but it might give you a starting point. The motherboard's on-board graphics chip is a GeForce 8300 which supports DXVA for video acceleration, so playing Full HD video content has not been a problem. It also has an HDMI connector. The case I'm using only makes one expansion slot available, so if you need both PCI audio and a TV decoder card you'll need a bigger one. I'm using a Squeezebox for audio, so I can get by with the one slot for TV. This is my second Silverstone case and I'd buy a third one if I was building again, they're pricey but the build quality and layout has been very good in my experience, and they look good enough to blend in with existing AV equipment. With the cooler mentioned it's also quiet enough that we don't notice it among the fridge etc., but not completely silent if that's what you need (for that you'll probably need a special case/PC tailored for recording studio use, for instance).

24/7 uptime was not a problem for me running 64-bit Vista Home Premium, nor has it been after upgrading to Windows 7.

May I add questions? Anyone having experience with quiet Notebooks? I'm rather interested in those, because of the display. I want to upgrade to a stronger notebook... Doesn't have to be a new one, because it only needs to do audio.

My current setup is:- IBM Thinkpad T22 (its somewhere in the 800mhz range) with SB-Card for PCMCIA- XP, fb2k in fullscreen and Girder for a lot of fun in remote controlling.

May I add questions? Anyone having experience with quiet Notebooks? I'm rather interested in those, because of the display. I want to upgrade to a stronger notebook... Doesn't have to be a new one, because it only needs to do audio.

My current setup is:- IBM Thinkpad T22 (its somewhere in the 800mhz range) with SB-Card for PCMCIA- XP, fb2k in fullscreen and Girder for a lot of fun in remote controlling.

Take your laptop apart. I dont care if you dont have the tools or you're afraid: Buy them. Stop being a pussy, set aside the hour and a half and take your laptop apart. You will find the heatsinks caked with dust and hair. Clean them off and replace the thermal paste if you have any.

If you wanna get fancy replace a fan or two as well. Your laptop will run worlds better unless the heat has permanently damaged components which is somewhat rare.

When re-imaging xp in case thats something you want to do, look into the following terms: "nlite" "ryanVM update pack" and "BTS packs". Half a days research + Half a days work and your laptop will no longer crash.

i turned my old pc into something like that (but its more of a file/web/ftp server with some desktop attached as well), what i did was new/cheap small housing, (took most of the HDs into the new machine), huge fan for the dual-core opteron, removed anything that had a propeler and replaced it with something without (GPU for example). The os is mint7 x64 (sort of ubuntu) and then i somehow managed to get cool and quiet working, so the cpu sits at 1Ghz most of the time..., of course no avisynth here and i didnt really bother with sound at all (i know its working, but thats all). This new nvidia passively cooled gpu is still good enough to make some use of those zexy compiz stuff

If you strive for low power consumption and quietness, you definitely should consider an SSD.Contrary to what an earlier poster said, they do not cost a fortune (depending of course howvast your music library is).According to Amazon 128 GB are available for not much more than 200 Euros.(Of course this one will not be very fast, but for a music library it should be sufficient)